Nitroxyl spin probe studies often result in superimposed electron paramagnetic resonance (EPR) spectra representing the different environments probed by the nitroxyl radical. It has been observed that addition of a paramagnetic metal to the system can broden the EPR signal of the nitroxyl radical in one of the environments, thus facilitating observation of nitroxyl in the other environment. Among the paramagnetic metal species which have been found useful for this purpose, the advantages of Fe(CN) 63 minus have been emphasized. Using theoretical results as a guide we have found that Cr(oxalate) 33 minus and Cr(SCN) 63 minus are more effective than Fe(CN) 63 minus. It is proposed to develop further reagents to optimize the broadening of nitroxyl spin probes and spin labels by paramagnetic metals. A wide variety of brodening reagents will be required for the diversity of applications now evident.